Why do textures stick around in the editor?
To get rid of unused textures, you have to save and then reload your file. 

Either use "File - Open Recent - " or hit Shift+Ctrl+O and then hit Enter, since your latest save is usually in the first place of the list. 

If you want to get rid of a texture despite having it assigned, press Shift and hit the X button beside the texture's name in the UV/image editor Texture dropdown. 

Besides that, textures are kept around on purpose. This comes in handy when you are optimizing your workflow. And you are certainly able to unwrap a model without any texture at all.